Duck lettuce (Ottelia alismoides)

General information
It is annual water plant and it has similar leaf to Plantain (see the picture right).  Its leaf has long petiole or leaf stalk and green-brown or purple-brown color. The shape of leaf is curly and oval or wide oval with irregular teeth and tusks at edge. The length of leaf varies widely from 10cm to 1m by living condition. It flowers only one day. The flower is white, pink or light purple bisexual flower, and the size of flower is from 2 to 3cm in diameter. It flowers on the water, individually in bract sheath in long stalk. It has obovate three petals, long oval three sepals and six stamens. After seed ripen, it will fall down in scattering on the water, and floating for a while until reaching to next place to start living.

It glows naturally in southeast Asia, north Africa, India and Australia. In Japan, it is found at paddy field, waterway, pond and wetland below Honshu and some islands such as Amami and Okinawa.

Duck lettuce is food in countries in southeast Asia and Okinawa island in Japan. Young leaves are soft and taste ordinary so it is edible raw after washing thoroughly. It is also good garnish of meat dishes. Please allow me to introduce some more information about edible water plants. Spirogyra is a kind of algae which is eaten in peculiar area as filling of omelet, soup as well as raw diet. Swamp cabbage is famous in Chinese dishes. I love Pak-bung faidaeng (Stir-fried Morning Glory with Chili); this is a Thai dish made with it. Duck lettuce is not only known as food but also herb medicine. It is said to be effective for cough fever and diuretic.
Water plant can take in nitrogen and phosphorous in water easily and grow up quickly. As a result it can help water purification.

Hard water

Generally, water that requires considerable amounts of soap to produce a foam or lather and that also produces scale in hot water pipes, heaters, boilers, and other units in which the temperature of water is increased materially.With respect to hardness, waters have been classified as follows: 0–25 mg/L as calcium carbonate (CaCO3), very soft; 25–75 mg/L as CaCO3, soft; 75–150 mg/L as CaCO3, moderately hard; 150–300 mg/L as CaCO3, hard; 300 mg/L as CaCO3 and up, very hard. (by “AWWA Drinking Water Dictionary”)


Generally, lime is calcium oxide (CaO) or /and slaked lime = hydroxide (Ca(OH)2 ) (by “Water supply terms dictionary”, Japan)

Lime–soda ash softening

A water treatment that makes use of lime softening followed by a reduction of noncarbonate hardness by the addition of soda ash (Na2CO3) to form an insoluble precipitate that is removed by filtration. This method of removing
hardness by precipitation is sometimes used by municipalities, but it will leave 85 mg/L or more of residual hardness as calcium carbonate (CaCO3).
(“AWWA Drinking Water Dictionary”)

Lime softening

The process of removing water hardness by adding lime to precipitate solids composed of metal carbonates and hydroxides. Clarification may or may not also occur. (“AWWA Drinking Water Dictionary”)


Microcyctine are produced by Microcystis, Mycrocystine is one of poi sonic algae. It is a parameter to be considered in Japan Water Quality Standard for Drinking Water. Its target level is less than 0.0008 mg/l.

Water softening

The removal of calcium and magnesium ions, which are the principal causes of hardness in water. The cation exchange resin method is most commonly used for residential and commercial water treatment. In municipal and industrial water treatment, the process can be lime softening or lime–soda ash softening, called precipitative softening. (“AWWA Drinking Water Dictionary”)